RE: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi_safe()

Kaplan, David
2025-10-20T14:50:02Z

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Peter Zijlstra <peterz@infradead.org>
> Sent: Thursday, October 16, 2025 9:48 AM
> To: Kaplan, David <David.Kaplan@amd.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>; Borislav Petkov <bp@alien8.de>; Josh
> Poimboeuf <jpoimboe@kernel.org>; Pawan Gupta
> <pawan.kumar.gupta@linux.intel.com>; Ingo Molnar <mingo@redhat.com>; Dave
> Hansen <dave.hansen@linux.intel.com>; x86@kernel.org; H . Peter Anvin
> <hpa@zytor.com>; Alexander Graf <graf@amazon.com>; Boris Ostrovsky
> <boris.ostrovsky@oracle.com>; linux-kernel@vger.kernel.org
> Subject: Re: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi_safe()
>
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> On Thu, Oct 16, 2025 at 02:40:51PM +0000, Kaplan, David wrote:
>
> > > Can we please keep this in sync_core()? Something like:
> > >
> > > static __always_inline void sync_core(void)
> > > {
> > > if (static_cpu_has(X86_FEATURE_SERIALIZE)) {
> > > serialize();
> > > return;
> > > }
> > >
> > > + if (repatch_in_progress) {
> > > + sync_core_nmi_safe();
> > > + return;
> > > + }
> > > +
> > > iret_to_self();
> > > }
> > >
> > > That way all the modern stuff that has SERIALIZE will still use that.
> >
> > Hmm, I can't quite do that because sync_core() is used in a number of
> > other places too (unless we make repatch_in_progress a true global).
>
> We could just out-of-line the thing; nothing using this should care
> about cycles -- all of this is quite expensive.
>

Coming back to this, are you thinking we should just create something like 'text_poke_sync_core()' inside alternative.c and that can use:
1. SERIALIZE (if available)
2. MOV-CR2 (if re-patching)
3. Else, IRET

And maybe someday we put MFENCE into there too for AMD parts.

Right now, of course this is the only logic that would care about an NMI-safe sync_core(). So maybe this makes sense vs creating a generic version that nobody else is using?

--David Kaplan